Methods for reducing the curvature in boron-doped silicon micromachined structures

ABSTRACT

Layers of boron-doped silicon having reduced out-of-plane curvature are disclosed. The layers have substantially equal concentrations of boron near the top and bottom surfaces. Since the opposing concentrations are substantially equal, the compressive stresses on the layers are substantially balanced, thereby resulting in layers with reduced out-of-plane curvature.

This is a continuation of application Ser. No. 09/634,932, now U.S. Pat.No. 6,544,655, filed on Aug. 8, 2000, entitled “METHODS FOR REDUCING THECURVATURE IN BORON-DOPED SILICON MICROMACHINED STRUCTURES.

FIELD OF THE INVENTION

The present invention is related generally to semiconductormanufacturing and Micro Electro Mechanical Systems (MEMS). Morespecifically, the invention relates to methods for reducing thecurvature of a boron-doped silicon layer.

BACKGROUND OF THE INVENTION

Micro Electro Mechanical Systems (MEMS) often utilize micromachinedstructures such as beams, slabs, combs, and fingers. These structurescan exhibit curvature due to internal stresses and doping gradients. Thecurvature can be a significant source of error in inertial sensors suchas accelerometers and gyroscopes. Many desired structures have aflatness design criteria that is difficult or impossible to achieveusing current processes. In particular, silicon layers heavily dopedwith boron can have a significant curvature when used in suspendedstructures.

The aforementioned structures are often made starting with a siliconwafer substrate. A boron-doped silicon epitaxial layer is then grown onthe silicon wafer substrate and is subsequently patterned in the desiredshape. As is further described below, the boron is used as an etch stopin later processing to allow the easy removal of the silicon substrate,leaving only the thin boron-doped epitaxial layer.

At the interface between the boron-doped epitaxial layer and the siliconsubstrate, the boron tends to diffuse out of the epitaxial layer andinto the silicon substrate. This depletes the epitaxial layer of someboron, and enriches the silicon substrate with boron. The epitaxiallayer thus often has a reduced concentration of boron near theinterface, which is sometimes called the “boron tail.”

After the boron-doped silicon epitaxial layer has been grown to thedesired thickness, or at some later point of processing, the siliconsubstrate is removed often using an etchant that is boron selective.Specifically, the etchant will etch away the silicon substrate, but notthe boron-doped silicon epitaxial layer. One such etchant is a solutionof ethylene diamine, pyrocatechol, and water (EDP). The etchanttypically etches the silicon at a fast rate up to a certain high levelboron concentration, at which point the etch rate significantly slows.This high boron concentration level is termed the etch stop level.

The boron concentration near the epitaxial layer surface having theboron tail may be lower than the etch stop level, allowing the etchingto remove some of the epitaxial layer surface at a reasonable rate,stopping at the etch stop level of boron concentration beneath theinitial surface. The resulting boron-doped structure, such as a beam,thus has two surfaces, the silicon side surface that has the boron tailand the air side surface that has a boron surface layer concentrationsubstantially equal to the concentration in the bulk of the beam awayfrom either surface. Thus, the opposing surfaces have different boronsurface layer concentrations.

Boron occupies substitutional lattice sites in silicon, the boron havinga Pauling's covalent radius roughly 25% smaller than that of silicon.The size difference causes the boron-doped layers to shrink relative tothe undoped or lower doped layers. This size difference leads to aninitial tensile stress, with higher boron concentrations leading tohigher tensile stresses and lower boron concentrations leading to lowertensile stresses. After release from the substrate, the lower boronconcentrations in the tail results in a relatively lower tensile stressthan the tensile stress in the air side layer having a higher boronconcentration. The tensile stress can transition to a compressive stressafter further process steps, such as oxidation and annealing at hightemperatures. Regardless of the exact mechanism, an unequal surfacelayer boron concentration in silicon can lead to an unequal applicationof stress by those layers in the structure which can lead to the cuppingor out-of-plane bending and curvature of a structure where flatness isdesired.

What would be desirable, therefore, is a process for reducing theunequal surface layer concentrations of boron in boron-doped silicon toproduce substantially flat or planar boron-doped siliconmicrostructures.

SUMMARY OF THE INVENTION

The present invention provides methods for forming relatively planarboron-doped silicon layers having reduced out-of-plane curvature byproviding substantially balanced doping profiles of boron near each ofthe layer surfaces. A boron-doped silicon epitaxial layer is first grownon a silicon substrate, causing the boron near the silicon substrate todiffuse out of the epitaxial layer into the silicon substrate. As in theprior art, this depletes the boron concentration near the interfacebetween the epitaxial layer and the silicon substrate. However, and in afirst illustrative embodiment of the present invention, a secondepitaxial layer is grown on the first boron-doped silicon epitaxiallayer. The second epitaxial layer preferably has a boron concentrationthat is less than the boron concentration in the first grown epitaxiallayer. Thus, boron in the first boron-doped epitaxial layer tends todiffuse into both the silicon substrate and the second epitaxial layer.This creates substantially similar “boron tails” at both surfaces of thefirst epitaxial layer. A boron selective etch can be used to remove boththe silicon substrate and the second epitaxial layer. Since theremaining first epitaxial layer has substantially similar “boron tails”at both top and bottom surfaces, the compressive stresses aresubstantially balanced leaving a relatively planar layer.

It is contemplated that any suitable material can be used to deplete theboron concentration near the top surface of the first boron-dopedepitaxial layer. For example, rather than growing a silicon based secondepitaxial layer, it is contemplated that an oxide layer may be used.Preferably, the oxide layer is selected such that the boron segregatedinto the oxide layer, depleting the surface silicon layer of boron. Onesuitable oxide layer is silicon oxide that can be formed through theoxidation of the silicon in the expitaxial layer.

Rather than growing a boron-doped first epitaxial layer on the siliconsubstrate, it is contemplated that the top surface of a silicon wafermay be directly doped with boron by, for example, diffusion, ionimplantation, or any other suitable method. Then, the second epitaxiallayer may be grown directly on the top surface of the silicon wafer. Asdescribed above, the boron may tend to diffuse both into the substrateand into the second epitaxial layer, leaving substantially similar“boron tails” on both sides of the heavily doped silicon layer. A boronselective etch can then be used to remove both the low-boron-dopedsilicon substrate and the second epitaxial layer.

Instead of forming substantially similar “boron tails” on either side ofa heavily boron-doped layer to reduce the curvature of the layer, thepresent invention also contemplates providing a layer with a boron tailnear one surface, and then substantially removing the boron tail. Inthis embodiment, a first boron-doped silicon epitaxial layer may begrown on a silicon substrate. Alternatively, and as indicated above,boron may be provided directly in the top surface of the siliconsubstrate. In either case, the boron tends to diffuse into the siliconsubstrate, thereby creating a boron tail. The silicon substrate can beetched using a first etchant for a first period of time, such that thesilicon substrate and at least part of the boron tail are removed at afirst etch rate. The silicon substrate can then be further etched usinga second etchant for a second period of time, such that more of theboron tail is removed at a second etch rate. The second etchant can bethe same as the first etchant or a different etchant that is lessinhibited by boron.

In a related method, it is contemplated that the second etchant may benon-boron selective etchant, such as a dry etch (RIE). In thisembodiment, the first etchant, which is boron selective, can be used toremove the silicon substrate and at least part of the boron tail up theetch stop level. The non-boron selective etchant is then used to removethe remaining boron tail, or any portion thereof. The non-boronselective etch may also etch away some of the material from the oppositeside surface of the heavily boron-doped layer.

Another method contemplates providing a relatively planar wafer having aheavily boron-doped layer thereon. In this illustrative embodiment, afirst heavily boron-doped epitaxial layer is grown on a top surface of asilicon wafer, followed by a second non-doped (or lightly doped)epitaxial layer. Because of the tensile stress in the boron doped layer,the wafer will show a significant curvature (cupping). In order toreduce wafer curvature so that it is compatible with further processes,another heavily doped epitaxial layer is grown on the bottom surface ofthe wafer. In many cases, during the epitaxial growth of the borondoped, silicon epitaxial layer from the bottom side of the wafer, a thinboron doped layer is grown on the front side of the wafer as well. A dryetch (non-boron selective etch) is then used to remove the heavily dopedepitaxial layer on the top surface of the structure, and possibly partof the non-doped (or lightly doped) epitaxial layer thereunder. A boronselective etch is then used to remove the remaining portion of thenon-doped (or lightly doped) layer on the top surface of the structure.This may produce a relatively planar wafer because heavily boron-dopedlayers remain on both the top and bottom surfaces of the wafer. It hasalso been found that the top surface of the top heavily boron-dopedlayer has very few defects with little contamination, thereby providingan ideal layer for forming the desired micromachined structures such asbeams, slabs, combs, and fingers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of boron atom concentration versus depth into aboron-doped silicon layer, showing the boron tail having decreasingboron atom concentration toward the silicon substrate side;

FIGS. 2A-2D are schematic representations of a method for creating arelatively planar boron-doped silicon epitaxial layer having a borontail near both top and bottom surfaces, the method including the growthof a second, lower boron-doped silicon epitaxial layer over a firstboron-doped silicon epitaxial layer;

FIGS. 3A-3D are schematic representations of the method of FIGS. 2A-2D,in greater detail;

FIG. 4 is a transverse cross-sectional view of the three layers of FIG.3C illustrating the boron tail near both top and bottom surfaces of theboron-doped epitaxial layer;

FIG. 5 is a transverse cross-sectional view of the boron-doped epitaxiallayer of FIG. 3D illustrating the boron tail near both top and bottomsurfaces of the boron-doped epitaxial layer;

FIGS. 6A-6D are schematic representations of a method for creating arelatively planar boron-doped silicon epitaxial layer having a reducedor eliminated boron tail, the method including increased etching of theboron-doped silicon epitaxial layer in the region of the boron tail;

FIG. 7 is a transverse cross-sectional view of the two layers of FIG.6B, illustrating the boron tail near the silicon substrate side surfaceof the boron-doped epitaxial layer, as well as the initial etch-stoplevel and the extended etch-stop level;

FIGS. 8A-8D are schematic representations of a method for creating aboron-doped silicon epitaxial layer having a boron tail near both topand bottom surfaces, the method including the growth of an oxide layerover the boron-doped silicon epitaxial layer;

FIGS. 9A-9D are schematic representations of a method for creating aboron-doped silicon epitaxial layer having a reduced or eliminated borontail, the method including dry etching both sides of the boron-dopedsilicon epitaxial layer, including the side having the boron tail;

FIG. 10 is a schematic transverse cross-sectional view of a microstructure having a non-curved planar cantilever formed from aboron-doped silicon epitaxial layer; and

FIGS. 11A-11E are schematic representations of a method for creating asubstantially planar wafer with low defect densities in the top surfaceof a highly boron-doped top layer.

DETAILED DESCRIPTION OF THE INVENTION

The formation of layers used to create structures such as MEMSmicrostructures often includes the growth of a boron-doped siliconepitaxial layer upon a single crystal silicon substrate. The boron isused as an etch stop in later processing to allow the easy removal ofthe silicon substrate, leaving only the thin boron-doped epitaxial layerto obtain the final resulting microstructure. The boron-doped epitaxiallayer typically has an “air side” and a “silicon substrate side.” Theconcentration of boron can remain relatively constant from the center ofthe layer to the air side surface. At the silicon substrate side,however, the boron concentration drops off, as some of the borondiffuses out of the boron-doped silicon epitaxial layer and into thesilicon substrate layer. This drop off in boron concentration is knownas the “boron tail.”

FIG. 1 is a graph 20 of boron atom concentration versus depth into aboron-doped silicon epitaxial layer, showing the boron tail havingdecreased boron atom concentration toward the silicon substrate side.The X axis corresponds to the depth of the layer, from the air sidesurface, indicated at 28, to the silicon substrate side surface,indicated at 30. The plot includes a constant boron concentration regionat 22, a shoulder region at 24, dropping to an etch stop level indicatedat 26, and dropping further below the etch stop level as indicated inregion 27. In one method, a constant boron concentration of about1.5×10²⁰ boron atoms per cubic centimeter is provided in the constantboron concentration region.

When using a boron selective etch such as EDP, the silicon substratesurface will be etched away, as well as region 27, but with the etchstopping near etch-stop level 26, leaving the remainder of the epitaxiallayer substantially intact, and forming a boron-doped silicon layer.Etching may not stop completely at the etch stop level, but may slowconsiderably. With some etchants, an etch stop level occurs at about 7to 9×10¹⁹ boron atoms per cubic centimeter. At this level and above, theetching rate drops one to two orders of magnitude for some etchants suchas EDP.

A first method for reducing the curvature of a boron-doped structureincludes encapsulating the air side of the epitaxial layer in a secondepitaxial boron-doped silicon layer having a significantly lower boronconcentration than the first epitaxial layer. After the growth of thefirst boron-doped epitaxial layer to the desired thickness, the growthprocess can be continued with the growth of a layer of silicon having alow or no concentration of boron. The concentration of boron ispreferably at least one order of magnitude lower than the etch stoplevel for the etchant to be used. In one embodiment, the secondepitaxial layer is very thin, between about 2 and 10 microns.

After the desired thickness of the second epitaxial layer has beenachieved, the growth can be stopped. In one embodiment, the wafer iskept at the same temperature for a period of time about equal to thegrowth time of the first epitaxial layer. A second boron tail can thusbe formed toward the second epitaxial layer. The wafer can be etched inan etchant such that the lower boron concentration silicon layers onboth sides of the first epitaxial layer are removed. In one embodiment,EDP is used as the etchant, and the etching stops at a boronconcentration of about 9×10¹⁹ cm⁻³, leaving the air side of the waferhaving a boron tail similar to the boron tail in the silicon substrateside. If desired, the resulting wafer may be polished to remove thesurface roughness which can be associated with the dislocation linesformed in silicon as a result of the high stress induced by the borondoping.

FIGS. 2A-2D include schematic representations of a first method forcreating a boron-doped silicon epitaxial layer having a boron tail nearboth surfaces, the method including the growth of a second, lowerconcentration boron-doped silicon epitaxial layer over the first, higherconcentration boron-doped silicon epitaxial layer. Beginning with FIG.2A, a silicon wafer 30 is provided as a substrate, which is laterremoved through etching. Silicon wafer 30 has a first surface 32 and asecond surface 34. A first epitaxial layer 36 of boron-doped silicon isgrown on silicon wafer 30 on first surface 32, as illustrated in FIG.2B. The growth of first epitaxial layer 36 forms an interface 42 betweenfirst epitaxial layer 36 and silicon wafer 30. Boron tends to diffuseout of first epitaxial layer 36 near interface 42, from first epitaxiallayer 36 into silicon wafer 30. First epitaxial layer 36 extends betweena silicon side surface 40 and an air side surface 38.

FIG. 2C illustrates another step in the process involving the growth ofa second boron-doped silicon epitaxial layer 44 on first boron-dopedepitaxial layer 36. Second epitaxial layer 44 has a lower (or no)concentration of boron than first epitaxial layer 36 and forms a secondinterface 50 between the two epitaxial layers. Due to the lowerconcentration of boron in the second epitaxial layer, boron tends todiffuse from the first epitaxial layer 36 into the second epitaxiallayer 44. The out diffusion of boron from first epitaxial layer 36creates a second boron tail near the air side 38. This second boron tailis preferably similar to the boron tail formed near silicon side 40,ultimately creating similar surface regions of stress gradient relativeto the bulk of the epitaxial layer, at both surfaces of the firstepitaxial layer 36.

FIG. 2D illustrates first epitaxial layer 36 after etching with a boronselective etchant, for example, EDP. Both silicon wafer 30 and secondepitaxial layer 44 are removed, leaving the first epitaxial layer 36with surface regions of reduced boron concentration at both silicon side40 and air side 38.

Referring now to FIGS. 3A through 3D, the first method discussed, withrespect to FIGS. 2A through 2D, is illustrated in greater detail. FIGS.3A through 3D correspond to FIGS. 2A through 2D, respectively, andinclude many of the same reference numerals which need not beidentified. FIG. 3B illustrates the formation of interface 42 betweensilicon wafer 30 and first epitaxial layer 36, indicated by wavy crosshatching, including a boron depleted region 41 first epitaxial layer 36and a boron enriched layer 43 in silicon substrate 30. Arrow 48indicates the physical location that will correspond to the etch stopposition in first epitaxial layer 36.

FIG. 3C illustrates the formation of second interface region 50 betweensecond epitaxial layer 44 and first epitaxial layer 36, including aboron depleted region 54 and a boron enriched region 52, formed by theout diffusion of boron from the first epitaxial layer 36 to the secondlower boron concentration epitaxial layer 44. Arrow 56 indicates thephysical location that will correspond to the etch stop position on thesurface of first epitaxial layer 36 near interface 50.

FIG. 3D illustrates first boron-doped epitaxial layer 36 after etching,resulting in the loss of silicon wafer 30 and second epitaxial layer 44.Etching also results in the partial loss of the first epitaxial layer upto the etch stop positions, indicated by arrows 48 and 56. As can beseen in FIG. 3D, first epitaxial layer 36 has a boron tail near bothsurfaces, at 41 and 54. The boron tail regions preferably have similarboron concentration profiles and similar contributions of tensile stressapplied to each surface, acting to counterbalance the effect of theopposing tensile stress.

FIG. 4 is a transverse cross-sectional view of the three layers of FIG.3C, illustrating the boron tail near both surfaces of the boron-dopedepitaxial layer. A plot of boron concentration 59 is superimposed on thecross-sectional view. The composite layers include silicon wafer 30adjacent to first epitaxial layer 36 which is adjacent to secondepitaxial layer 44. First boron tail 41 may be seen near silicon layer30, and second boron tail 54 may be seen near second epitaxial layer 44.The boron selective etchant thus etches into first epitaxial layer 36 tothe etch stop locations on each surface of the layer. FIG. 5 is atransverse cross-sectional view of the boron-doped epitaxial layer 36 ofFIG. 3D after etching, illustrating the boron tails 41 and 54 near bothsurfaces of the boron-doped epitaxial layer.

Another method according to the present invention includes etching theboron tail for an extended period to reduce or eliminate the boron tail.At a boron concentration of about 7 to 9×10¹⁹ cm⁻³, the etch rate of theboron-doped silicon layers in several etching solutions, such as EDP(EPW), potassium hydroxide (KOH), and tetramethyl ammonium hydroxide(TMAH), decreases to varying degrees. For example, while EDP declinestwo orders of magnitude, the etch rate of TMAH declines by about afactor of five (5). Illustrative times for etching away the boron tailin different etchants are listed in Table 1 below.

TABLE 1 EDP KOH TMAH @ 90 C. 24% @ 60 C. 25% @ 70 C. Non-doped  0.5micron/min  0.5 micron/min  0.15 micron/min silicon Boron-doped 0.015micron/min 0.05 micron/min N/A silicon, 7 × 10¹⁹ cm⁻³ Boron-doped 0.002micron/min 0.03 micron/min 0.027 micron/min silicon, 10 × 20 cm⁻³ Timeto re- 70 to 500 min   33 min   37 min move the boron tail

By using such an extended etch in EDP, KOH, or TMAH, the boron tail canbe reduced, which reduces the curvature of the boron-doped structures.In this method, the epitaxial layer can be etched in EDP or anotheretching solution having high boron selectivity, up to the etch stoplimit. The sample can be kept in the same etching solution for a longerperiod of time, such as between about 30 minutes and several hours. Thisetch can be continued until the curvature is brought inside acceptablelimits. Alternatively, the sample may be moved into a second etchingsolution having a lower selectivity to boron doping, for example TMAH,for a second time period, such as about 30 minutes. This second etch canremove the silicon layer including the boron tail. It is contemplatedthat the curvature of the wafer can be measured at selected intervals oftime and the process adjusted accordingly.

FIGS. 6A-6D illustrate a second method for creating a boron-dopedsilicon epitaxial layer having a reduced or eliminated boron tail, themethod including increased etching of the boron-doped silicon epitaxiallayer, including the boron tail. The second method illustration requiresno figures similar to FIGS. 2A through 2D, which are omitted, but may beinferred. FIGS. 6A through 6D are similar in format to FIGS. 3A through3D, showing only a portion of each layer. FIG. 6A illustrates siliconsubstrate layer 30, and FIG. 6B illustrates the silicon substrate layerafter the growth of epitaxial layer 70, defining interface 42therebetween. A boron depleted tail region may be seen near interface 42including an inner tail region 78 and an outer tail region 80, whereinner tail region 78 has a boron concentration less than the bulk of theepitaxial layer, but greater than the outer tail region 80. The boronhas diffused into the boron enriched region 43 in silicon substratelayer 30, as previously described. The final dimensions of epitaxiallayer 70 are indicated by arrows. The limit of the first etch isindicated at 74, the limit of the second etch is indicated at 72, andthe final dimension of the air side surface of the epitaxial layer isindicated at 76.

FIG. 6C illustrates the result of a first etching step using a boronselective etchant. This first etching step is carried out under normalconditions in one embodiment. The first etching step results in theetching away of the silicon substrate and the epitaxial layer up to thefirst etch stop point where the boron concentration increases to a levelwhere the rate of etching significantly slows, often one or two ordersof magnitude below the rate at the outer surface. Thus, the firstetching step etches away part of epitaxial layer 70 to the pointindicated at 72, partially through the boron tail.

After the first etching step, a second etching step may be performed. Inone embodiment, the second etching step is a continuation of the firstetching step, with the second etching step being carried out for alonger than normal period. In one embodiment, the first etching step isperformed at a temperature of about 115 degrees Centigrade in a solventsuch as EDP for a period of about 500 minutes to remove the siliconwafer 30, which in one embodiment can have a thickness of about 500microns. The second etching step may then be performed at a temperatureof about 115 degrees Centigrade in a solvent such as EDP for a period ofabout 90 minutes. The second time period could be longer, to carry outthe etching of the higher boron concentration regions of the boron tail,due to the decreased etching rate at that higher boron concentrations.

In another embodiment, the second etching step is performed in anetchant different from the first etchant, such as KOH or TMAH. Thesecond etching step can be continued until the boron tail is evenfurther etched away, reducing the degree of curvature of the epitaxiallayer. FIG. 6D illustrates epitaxial layer 70 after the second etchingstep, having a greatly reduced boron tail at the silicon side of thelayer.

FIG. 7 is a transverse cross-sectional view of the two layers of FIG. 6Billustrating the boron tail near the silicon substrate side surface ofthe boron-doped epitaxial layer, as well as the initial etch-stop level81 and the extended etch-stop level 83. A plot 82 of boron concentrationis superimposed upon the epitaxial layer, with the extent of the firstetch indicated at 74 and the extent of the second, or extended etchindicated at 72. As can be seen from inspection of FIG. 7, the firstetching step etches away an outermost layer of epitaxial layer 70, andthe second etching step further etches away an outermost layer of theepitaxial layer. In one embodiment, the second etch step does notcompletely remove the boron tail, but significantly reduces thethickness, thereby significantly reducing the gradient stresscontributed by the boron tail. In some embodiments, in particular,embodiments utilizing less boron selective etchants, some of theopposing air side of the epitaxial layer is also somewhat removed.

FIGS. 8A-8D illustrate yet another method for creating a boron-dopedsilicon epitaxial layer having a boron tail near both top and bottomsurfaces. The method including the growth of an oxide layer over theboron-doped silicon epitaxial layer, rather than a silicon epitaxiallayer as shown in FIGS. 3A through 3D. This method is similar to themethod described with reference to FIGS. 3A through 3D in that bothinclude forming a second layer over the boron-doped silicon layer forthe purpose of drawing out some boron from the epitaxial layer bydiffusion into the second layer. The out diffusion of boron causes theformation of a second boron tail on the second surface of the epitaxiallayer to counteract the effects of the first boron tail caused by thediffusion of boron out of the epitaxial layer and into the siliconlayer.

When forming the oxide layer, the boron tends to segregate into theoxide layer. While the boron is segregating into the oxide layer, boronwill likely continue to diffuse into the silicon layer. Thus, it isoften beneficial to have the segregation coefficient of boron in theoxide layer be higher than the diffusion rate of the boron in thesilicon substrate layer. Attempting to achieve a pair of relativelybalanced boron tails may thus benefit from a selection of materials,thicknesses, times and temperatures in forming the oxide layer. In oneembodiment, a silicon oxide layer of about 0.5 microns thick of wetoxide is grown at about 1000 degrees Centigrade.

FIG. 8B illustrates the formation of interface 142 between silicon wafer130 and first epitaxial layer 136, indicated by wavy cross hatching,including a boron depleted region 141 in first epitaxial layer 136 and aboron enriched layer 143 in silicon substrate 130. Arrow 148 indicatesthe physical location that will correspond to the etch stop position infirst epitaxial layer 136.

FIG. 8C illustrates the formation of second interface region 150 betweena second oxide layer 144 and the first epitaxial layer 136, including aboron depleted region 154 and a boron enriched region 152, formed by thesegregating of boron from first epitaxial layer 136 to second, oxidelayer 144. Arrow 156 indicates the physical location that willcorrespond to the etch stop position on the surface of first epitaxiallayer 136 near interface 150.

FIG. 8D illustrates first boron-doped epitaxial layer 136 after etching,resulting in the loss of silicon wafer 130 and second oxide layer 144.Etching also results in the partial loss of the first epitaxial layer upto the etch stop positions, indicated by arrows 148 and 156. As can beseen in FIG. 8D, first epitaxial layer 136 has a boron tail on bothsurfaces, at 141 and 156. The boron tail regions preferably have similarboron concentrations and similar contributions to the stress gradientapplied to each surface, tending to counterbalance the effect of theopposing stress profile. In some embodiments, the resulting boronconcentration in the final boron-doped epitaxial layer is similar tothat illustrated in FIG. 5.

Referring now to FIGS. 9A through 9D, yet another method for reducingout-of-plane curvature in a boron-doped silicon epitaxial layer isillustrated. The method includes dry etching both surfaces of theboron-doped silicon epitaxial layer, including the surface having theboron tail and the surface not having the tail. The method illustratedin FIGS. 6A through 6C can be somewhat similar to the method illustratedin FIGS. 9A through 9C. In one embodiment, the wafer is first etched inEDP up to the etch stop level, then rinsed and dried. The wafer is thenplaced in a dry etch (such as a Reactive Ion Etch), and up to about 1micron of silicon is removed from the entire structure.

FIG. 9A illustrates silicon substrate layer 130, and FIG. 9B illustratesthe silicon substrate layer after the growth of epitaxial layer 170,defining interface 142 therebetween. A boron depleted tail region may beseen near interface 142 including an inner tail region 178 and an outertail region 180, where inner tail region 178 has a boron concentrationless than the bulk of the epitaxial layer, but greater than the outertail region 180. The boron has diffused into the boron enriched region143 in silicon substrate layer 130, as previously described. The finaldimensions of epitaxial layer 170 are indicated by arrows. The limit ofthe first etch is indicated at 174, the limit of the second etch isindicated at 172, and the final dimension of the air side surface of theepitaxial layer is indicated at 176.

FIG. 9C illustrates the result of a first etching step using a boronselective etchant. This first etching step can be carried out undernormal conditions in one embodiment. The first etching step results inetching away of the silicon substrate and the epitaxial layer up to thefirst etch stop point where the boron concentration increases to a levelwhere the rate of etching significantly slows, often one or two ordersof magnitude below the rate at the outer surface. The first etching stepetches away epitaxial layer 170 to the point indicated at 174, partiallythrough the boron tail. As can be seen from inspection of FIG. 9C, thefinal dimension indicated at 176 is disposed below the air side surfaceof epitaxial layer 170.

FIG. 9D illustrates the results of a dry etch to remove more or all ofboron tail region 172. The dry etch may be non-boron selective, such asa Reactive Ion Etch. Thus, the dry etch step can be used to removematerial from both the top and bottom surfaces of the epitaxial layer.As indicated by FIG. 9D, boron tail 172 is significantly or totallyremoved.

Referring now to FIG. 10, a microstructure device 200 is shown.Microstructure 200 represents a microstructure device, such as a MEMSdevice, incorporating a body 202 and a cantilevered beam 204 having atop surface 206 and a bottom surface 208. In one embodiment,microstructure 200 forms part of an accelerometer. Cantilevered beam 204is formed separately and later affixed to body 202 along a seam orinterface 210. Alternatively, beam 204 may be integrally formed withbody 202.

Beam 204 is preferably flat, having very little out-of-plane curvature.Accelerometers and other microdevices can benefit from planar structureshaving very flat surfaces and/or require ends that are centered relativeto another part of the device. The present invention can be used toprovide such components with very little out-of-plane curvature, therebyimproving the performance of such microdevices.

FIGS. 11A-11E are schematic representations of a method for creating asubstantially planar wafer with low defect densities in the top surfaceof a highly boron-doped top layer. Beginning with FIG. 11A, a siliconwafer 250 is provided as a substrate. Silicon wafer 250 has a firstsurface 252 and a second surface 254. A first epitaxial layer 256 ofboron-doped silicon is grown on the first surface 252 of the siliconwafer 250, followed by a second non-doped (or lightly doped) epitaxiallayer 258, as illustrated in FIG. 11B. Because of the increased tensilestress caused by the boron-doped epitaxial layer 256, the silicon wafer250 may begin to cup out-of plane.

To reduce the out-of-plane curvature of the wafer 250, another heavilydoped epitaxial layer is grown on the bottom surface of the wafer. Insome cases, the growth of the boron-doped epitaxial silicon layers onthe bottom side of the wafer results in the parasitic deposition of athin layer of boron-doped silicon layer on the top side of the siliconwafer as well, as shown at 260 and 262 in FIG. 11C. Then a dry etch(non-boron selective etch) is used to remove the heavily doped epitaxiallayer 260 on the top surface of the structure, and possibly part of thenon-doped (or lightly doped) epitaxial layer 258, as shown in FIG. 11D.Finally, a boron selective etch (EDP) is then used to remove theremaining portion of the non-doped (or lightly doped) layer 258 of FIG.11D, as shown in FIG. 11E. The resulting structure includes heavilyboron-doped layer 256 and 262 on both the top and bottom surfaces of thewafer, which may reduce the out-of-plane curvature of the wafer 250. Ithas also been found that the top surface of the top heavily boron-dopedlayer 256 may have very few defects with little contamination, therebyproviding an ideal layer for forming the desired micromachinedstructures such as beams, slabs, combs, and fingers.

In all of the above embodiments, it is contemplated that the firsthighly doped epitaxial layer may be replaced by directly doping the topsurface of a silicon wafer. That is, boron may be provided directly intothe top surface of the silicon wafer by diffusion, ion implantation, orany other suitable method to produce a highly boron-doped layer. Theremaining steps may remain substantially unchanged.

Numerous advantages of the invention covered by this document have beenset forth in the foregoing description. It will be understood, however,that this disclosure is, in many respects, only illustrative. Changesmay be made in details, particularly in matters of shape, dimension, andarrangement of parts without exceeding the scope of the invention. Theinvention's scope is, of course, defined in the language in which theappended claims are expressed.

1. A wafer comprising: a silicon substrate having a first surface and a second surface; a first epitaxial layer, having a first thickness and a first concentration of boron doping, deposited on the first surface of said silicon substrate; and a second epitaxial layer, having a second thickness and a second concentration of boron doping, deposited on the second surface of said silicon substrate; and wherein: the first epitaxial layer exerts a first stress on the substrate at the first surface; the second epitaxial layer exerts a second stress on the substrate at the second surface; the first stress is approximately equivalent to the second stress; and the wafer has minimized curvature.
 2. The wafer of claim 1, wherein the first and second thicknesses and the first and second concentrations of doping are selected to minimize curvature of the wafer.
 3. The wafer of claim 2, wherein the first and second thicknesses may be non-equal.
 4. The wafer of claim 3, wherein the first and second concentrations may be non-equal.
 5. The wafer of claim 1 wherein: a combination of the first thickness and the first concentration of doping provides the first stress on the first surface of said substrate; a combination of the second thickness and the second concentration of doping provides the second stress on the second surface of said substrate; and the first and second stresses minimize curvature of the wafer.
 6. The wafer of claim 5, wherein the wafer has a balanced stress profile.
 7. The wafer of claim 1, wherein the first and second concentrations of doping result in an unbalanced doping profile.
 8. The wafer of claim 7, wherein the first and second thicknesses may be unequal.
 9. The wafer of claim 1, wherein said first or second layer may be partially etched to minimize curvature of the wafer.
 10. A wafer comprising: a substrate having having first and second surfaces; a first layer, having a first thickness and a first concentration of doping, formed on the first surface of said substrate; and a second layer, having a second thickness and a second concentration of doping, formed on the second surface of said substrate; and wherein: the first layer applies a first stress on the substrate at the first surface; the second layer applies a second stress on the substrate at the second surface; and the water has miniaturized curvature.
 11. The wafer of claim 10, wherein at least one of the first and second concentrations of doping is adjusted to minimize curvature of the wafer.
 12. The wafer of claim 10, wherein at least one of the first and second thicknesses is adjusted to minimize curvature of the wafer.
 13. The wafer of claim 12, wherein the first and second thicknesses may be non-equal.
 14. The wafer of claim 11, wherein the first and second concentrations of doping may be non-equal.
 15. The wafer of claim 14, wherein the wafer has a balanced stress profile.
 16. The wafer of claim 15, wherein: the substrate is silicon; and the doping is boron.
 17. A wafer comprising: a substrate layer having a first surface and a second surface; a first layer, having a first concentration of doping, deposited on the first surface of said substrate layer; and a second layer, having a second concentration of doping, deposited on the second surface of said substrate layer; and wherein: the first layer applies a first stress on the substrate layer at the first surface; the second layer applies a second stress on the substrate layer at the second surface; the first and second concentrations of doping have profiles that are not balanced; and the wafer has miniaturized curvature.
 18. The wafer of claim 17, wherein the first and second concentrations of doping are selected to reduce curvature of the wafer.
 19. The wafer of claim 18, wherein: the substrate layer is silicon; the doping is boron; and the first and second layers are epitaxial layers. 